The present invention relates to a method and apparatus for drying moisture-containing solids, such as domestic refuse and sludge cakes.
The invention is particularly useful, and is therefore described below, with respect to multi-purpose plants for processing domestic refuse and sludge cakes for some or all of the following purposes: (1) the disposal of the refuse and sludge cakes by incineration with efficient heat recovery and with non-polluting flue gases being discharged into the atmosphere; (2) the production of dried solid fuel and steam; (3) the utilization of a portion of the recovered heat to supply all the heat requirements for the drying operation, and for superheating the vaporized moisture to be used for the generation of all the power requirements of the plant; (4) the utilization of the remaining recovered heat to produce superheated steam for power generation or saturated steam for heating or other purposes; and (5) the production of fresh water by distillation from sea water, brackish water, polluted river water, or waste water, in a dual-purpose plant, where said superheated vaporized moisture (item 3) and said superheated steam (item 4), after being used for power generation, are further utilized to supply their heat of condensation for said fresh water production.
At the present time, disposing domestic refuse and sludge cakes is a very expensive operation. One method for disposing such refuse is by incineration, but refuse containing more than 50% moisture cannot be incinerated with the existing incinerators without auxiliary heating, especially if deodorized and non-polluted flue gases are to be discharged into the atmosphere. For example, in Paris, incinerators are supplied with 15 kg of fuel per ton of refuse for each 5% moisture content above 50%, as noted for example in the book "Les Residus Menagers", page 43 by Andre Saurin, 1967 (as well as other references mentioned below).
In the presently used methods of refuse and sludge cake incineration, the heat used to vaporize the moisture is generally a total loss because the produced water vapor is mixed with the drying hot gases. Furthermore, moisture reduces the flame temperature and increases the burning time, as the presence of the evaporated moisture increases the gas volume and thus reduces the concentration of the fuel and air in the burning mixture.
Thus, in the presently used methods the incinerator capacity and overall efficiency are considerably reduced by the moisture contained in the incinerated matter; also, the drying of the refuse or sludge cakes by direct contact heating with the combustion heating gases results in pollution of the overflowing cooled flue gases.
The present invention provides a method and apparatus having advantages in the above respects for drying moisture-containing solids, particularly domestic refuse and sludge cakes.
Very important and substantial advantages are provided when the invention is used for processing refuse and sludge cakes. Compare it for example with the method of disposing the Paris refuse in the manner described in the above-cited publication. According to that publication (page 43), the steam production for heating is 0.90 to 130 tons steam per ton of incinerated refuse, and the net average power production is 150 kWh per ton incinerated refuse. The average moisture content of Paris refuse is 30% (page 6 of the publication), so that the average power production and the average steam production for heating per ton of dry refuse are respectively, 150/0.70, or 214 kWh, and (0.90 + 1.30)/2 .times. 0.70, or 1.6 tons steam.
In sharp contrast to these figures, the present invention enables (as will be shown by Table I, item 12, and Table II, item 10, below) a power production of 640 kWh net and a steam production of 4.1 tons steam net, per ton of dry refuse. This result is obtained by drying and incinerating a refuse containing 60% moisture whereas with the existing incinerators, not only there is not produced any heat and power excess, but auxiliary heating is required.
This comparison to the prior art indicates that the present invention is a definite break-through in this field.